The hottest and coldest days of the year come roughly two days sooner than they did 50 years ago, according to a study published in Nature. The change coincides with the rise in global temperatures.

The seasons begin two days earlier than they did 50 years ago, a shift that may be related to human activity, according to researchers at UC Berkeley and Harvard University.

The season skewing means that the hottest and coldest days of the year come about two days sooner than they did 50 years ago, according to a study published in the Jan. 22 edition of the journal Nature. The study also found that the difference between average winter and summer temperatures shrank in the same 50-year span, indicating winters are heating up faster than summers.

The change coincides with the rise in global temperatures, which could suggest a link to human-induced global warming, said Alexander Stine, the study's first author and a graduate student at UC Berkeley's Department of Earth & Planetary Science.

"The pattern that we see suggests there's a relationship between global warming and the shifting of the seasons," Stine said.

Earlier seasons could affect farming, rainfall distribution, water supplies and the diversity of ecosystems, said Bill Patzert, a climatologist at NASA's Jet Propulsion Laboratory in La Canada Flintridge.

"When you see a shift like this, or what appears to be the beginning of a shift, it's yet another red flag about the potential implications of humans fiddling with the climate," he said.

An earlier spring could lengthen wildfire season in Western states and affect the availability of water resources, said Stephanie McAfee, a PhD student at the University of Arizona who studies the effect of climate change on ecosystems.

"We rely on water that falls as snow in the mountains that melts and delivers water to our reservoirs," she said. "The earlier that snow starts to melt, the less of a buffer we have."

Stine worked with Peter Huybers, assistant professor of earth and planetary sciences at Harvard, and Inez Fung, professor of atmospheric science at UC Berkeley. They studied global surface temperature measurements from 1850 through 1953, and 1954 through 2007. In the first period, land temperatures in the Northern Hemisphere peaked around July 21; in the later period, they peaked 1.7 days earlier.